Add support for importing existing wallets using an extended private
key (xprv) instead of a mnemonic phrase. This creates an HD wallet
that can derive multiple addresses, just like mnemonic-based wallets.
- Add hdWalletFromXprv() and isValidXprv() to wallet.js
- Add xprv type handling in getSignerForAddress()
- Create importXprv.js view with full validation and password flow
- Add import-xprv HTML view to index.html
- Wire up navigation from add-wallet view
- Enable +address button for xprv wallets in home view
Closes#20